Air Pressure Falling on a Six Percent Downgrade
You are driving a 3-axle tractor pulling a tandem-axle van at a gross combination weight of 78,000 lb down a 6 percent grade that runs for 5 miles. You selected a low gear at the crest and are holding 35 mph. To control speed you have made three firm brake applications of about 8 seconds each. At the top the dash gauge read 118 psi; it now reads 84 psi, and the low-air warning has not sounded. Engine speed is 1,800 rpm and the compressor is engine-driven. Two miles ahead the grade steepens into a 25 mph curve. The right shoulder is narrow with a guardrail, and a runaway truck ramp is one mile ahead on the right. A light rain is falling. Traffic behind you is light but closing.
1. What does the gauge trend tell you, and what should you do in the next few seconds?
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The gauge is the key clue: 118 psi fell to 84 psi after only three applications, and the compressor has not recovered the reserve. Normal governor cut-in is about 100 psi, so a healthy system should be back above 100 psi between applications. At 84 psi you still have braking, but the low-air warning must come on by 60 psi, and the emergency spring brakes can apply somewhere between 20 and 45 psi. You do not have enough margin to keep snubbing down a 6 percent grade to the 25 mph curve. Action: stop further brake applications, stay in the low gear, use the engine brake or retarder if equipped, and let the compressor build pressure. If speed cannot be held below 35 mph, or the gauge continues down, take the runaway ramp one mile ahead rather than risk the curve. Do not fan the brakes.
2. Why is it unsafe to pump or fan the brakes, and why not use the trailer hand valve to slow down?
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Each brake application uses air from the reservoir. Fanning or pumping gives no cooling benefit on a long grade and drains the reserve faster, which brings the spring brakes closer to an automatic application at the worst moment. The trailer hand valve applies only the trailer service brakes. Using it while the tractor brakes are not balanced can swing the trailer out or jackknife the combination, and it also consumes air. With pressure already falling, the correct tool is engine braking and a lower gear, not more air applications.
3. After you stop, what inspection and pressure tests would you do before continuing?
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After stopping, secure the vehicle, chock the wheels, and inspect for a leak at the compressor, lines, hoses, and gladhands. Then run the CDL air-brake checks: build pressure to governor cut-out, shut the engine off, and check static leak with brakes released and applied; on a combination the allowable loss is about 3 psi per minute, not more. Confirm the low-air warning comes on by 60 psi, and confirm the spring brakes apply in the 20-45 psi range. Check that the compressor can build from 85 to 100 psi within 45 seconds. If the leak persists or build-up is slow, do not continue down the grade.